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Paper:
Bajekal, M, Scholes, S, O’Flaherty, M, Raine, R, Norman, PD and Capewell, S
(2013)
Implications of using a fixed IMD quintile allocation for small areas in
England from 1981 to 2007.
PLoS One, 8 (3). e59608.
1
Text S1: Implications of using a fixed IMD quintile allocation
for small areas in England from 1981 to 2007.
Supporting information for:
Unequal Trends in Coronary Heart Disease Mortality by Socioeconomic Circumstances, England 1982-2006: Analytical Study
Authors
Madhavi Bajekal1*, Shaun Scholes1, Martin O’Flaherty2, Rosalind Raine1, Paul Norman3, Simon Capewell2
1
Department of Applied Health Research, University College London, London, UK
2
Institute of Psychology, Health and Society, University of Liverpool, Liverpool, UK.
3
School of Geography, University of Leeds, Leeds, UK
Corresponding author: Madhavi Bajekal
*
2
Background
In this study we have used the composite score of the 2007 Index of Multiple Deprivation (IMD) to
categorise Lower Super Output Areas (LSOAs) into equal quintile groups of areas. The IMD is the government’s current preferred indicator of deprivation in England. Its main strength is that unlike
deprivation indices based on census data, the majority of the 33 indicators which underlie the
composite score can be updated between the inter-censal period using routinely collected data. The
IMD scores also provide a more granular and precise measure of local deprivation as they are based
on LSOAs which, unlike electoral wards, are statistical units designed to contain roughly equal sized
populations and capture similar 'neighbourhoods'. Furthermore, because LSOA boundaries remain
fixed over time, the distortions caused by frequent changes in the boundaries of alternative units of
aggregation are eliminated (e.g. electoral ward or local administrative districts) [1]. Hence, the index
was so designed to allow regularly updated IMD scores to be used to monitor the ‘real’ underlying
trends in area inequalities.
The IMD series was first released in 2000 (at electoral ward level). Subsequent updates in 2004, 2007
and 2010 were all produced at the LSOA level. The indices from 2004 onwards are highly
inter-correlated; this was expected as they use very similar, though not the same, datasets and methods.
For the purpose of our analysis of mortality trends from 1982 to 2006, we used the IMD score closest
to the end-point of our series – 2007 – to define our quintile groups with Q1 being the least deprived
and Q5 the most deprived areas. The IMD quintile group membership of an area and its boundary
remained fixed over the entire period of analysis, i.e. 1982-2006. This was partly for practical reasons
- there was no equivalent composite score of multiple deprivation for LSOAs available prior to 2004 -
and partly because the relative ranking of small areas in England is thought to have remained
remarkably stable over long periods whatever measure of relative deprivation is used [2].
Does area deprivation ranking remain stable over time?
Linking a time-series of health events to area types fixed for a single point in time is a common way
of analysing change over time. Since areas may change, this is recognised as less than ideal. We know
that over time some areas undergo ‘gentrification’ while others move down the deprivation ladder.
Selective (net) population migration between quintile groups is also likely to have an impact on the average ‘healthiness’ or otherwise of areas. However, aggregated over a large number of similar
LSOAs (c. 6,500 areas, with a combined population of about 6.5 million adults aged 35 and over) we
would expect that the net effect of moves of individual LSOAs on the margins between quintile
groups would have a minor impact. Hence, we tested our assumption of relative stability in quintile allocation across time and the potential scale of the ‘noise’.
Dr Paul Norman at the University of Leeds has previously analysed change in deprivation levels
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ward geography [3]. The aim of this analysis was to look at absolute change over the decade. We, on
the other hand, were interested in the stability or otherwise of the relative ranking in quintile
allocation of wards over time. We therefore requested Dr Norman to share with us the Townsend
scores he had calculated using data from three censuses – 1981, 1991 and 2001 – calculated on
common ward geography across all three time periods.
We normalised the deprivation score in each time period to the England average so that scores reflected the ward’s relative ranking at each time point. Unlike LSOAs, because wards were of
unequal population sizes (larger in inner-city deprived areas), we calculated population-weighted
quintile groups such that each quintile had equal fifths of the population, not equal numbers of areas.
On average, wards are about four times larger than LSOAs. In 2001 there were 7958 CAS (Census
Area Statistics) wards in England with an average population of 6,174 persons (min=557
max=35,102); in contrast there were 32,482 LSOAs with an average population of 1,513 persons
(min=1,000 max=6,537). Note there is a positive correlation of 0.88 (p<0.001) between Townsend
scores calculated for LSOAs using 2001 Census data and the IMD for 2007.
Our analysis of change in the relative position of wards, as allocated to deprivation quintiles, was
carried out using the Townsend deprivation scores for 1981, 1991 and 2001.
Results
Table 1 below shows the transition matrix of the percentages of the 1981 population as distributed
into deprivation quintiles based on 1981 score ranking and 2001 score ranking. Table 2 shows the
equivalent quintile matrix for the transition between 1991 and 2001 rankings for 1991 population.
Table 1: Transition matrix of % population distribution categorised into quintiles by 1981 and 2001 Townsend deprivation scores, England
1981
dep
ri
vat
ion
qui
n
ti
les
2001 deprivation quintiles
Q1 Q2 Q3 Q4 Q5 TOTAL
Q1 76 27 5 0 0 20
Q2 21 53 27 3 0 20
Q3 3 18 51 24 3 20
Q4 1 1 17 56 21 20
Q5 0 0 1 17 76 20
TOTAL 100 100 100 100 100 100 Summary
No change in quintile 76 53 51 56 76 62 Moved up 1+quintile 24 20 18 17 0 15 Moved down 1+quintile 0 27 31 27 24 22
In terms of population distribution, Table 1 shows that just over three-quarters (76%) of the
population in 1981 who were living in either in the least or the most deprived fifths of wards remained
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level remained unchanged. As we might expect, there was more movement in the intermediate
quintile groups (up and down). Hence, the population in wards that retained their deprivation group
categorisation over these two decades (i.e. 'on the diagonal') was 62%. The remainder were fairly
evenly split: 15% moved up one or more quintile groups ('gentrification'), and 22% moved down the
deprivation ladder.
Table 2: Transition matrix of % population distribution categorised into quintiles by 1991 and 2001 Townsend deprivation scores, England
1981 de p ri vat ion qui n ti les ( 1991 ?)
2001 deprivation quintiles
Q1 Q2 Q3 Q4 Q5 TOTAL
Q1 80 22 1 0 0 20
Q2 18 63 20 0 0 20
Q3 2 15 65 17 0 20
Q4 0 1 13 71 13 20
Q5 0 0 0 12 87 20
TOTAL 100 100 100 100 100 100 Summary
No change in quintile 80 63 65 71 87 73 Moved up 1+quintile 20 16 14 12 0 12 Moved down 1+quintile 0 22 21 17 13 15
The equivalent table for transitions between 1991 and 2001 is markedly more stable than that between
1981 and 2001 (Table 2). Changes in the 1980s included a large reduction in non-home ownership as people took advantage of the ‘right-to-buy’ their council rented property (39.6% non-home ownership
in 1981, 29.7% in 1991 and 28% in 2001). 80% of the 1991 population living in the most advantaged
quintile and 87% of those in the most deprived retained their respective position. Overall, about three-quarters (73%) of England’s population did not change quintile position between 1991 and 2001. Of
the remainder, 12% of the population moved up one or more quintile groups, and 15% moved down
the quintile grouping.
These results demonstrate that the top and bottom fifths of the deprivation distribution remained fairly
stable over the two decades of the study, particularly so at the extreme ends of the distribution
whereby in general non-deprived wards remain so as do deprived wards. However, as we might
expect, the consistency of the match deteriorated over time.
It is therefore plausible to assume that the quintile match between LSOAs over the 25-year period
would have been at least as stable as for CAS wards, and possibly more so.
Selective migration and it impact on inequality trends.
Numerous studies have shown that there is a consistent relationship between health and deprivation:
poorer areas have the worst health outcomes, with health improving as deprivation falls. This
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because of differences in the physical environment, resources and facilities between areas (or the
contextual effects). However it should be borne in mind that not all socially disadvantaged people live
in deprived areas, and vice-versa. Since area-based deprivation measures capture both the contextual
and compositional aspects of deprivation, they may be a more reliable measure of socioeconomic
circumstances than disadvantage measured between groups based on individual social position alone.
Recently, a number of studies examining the health-deprivation relationship have explored the
possibility that at least part of the explanation for the persistence of inequality relates to selective
migration of healthy people to less deprived areas and for either sicker people to move to poorer areas
or the relative immobility of sicker people relative to healthy out-migrants.
Norman and colleagues (2005)l used the closed population sample of the UK Longitudinal Study (LS)
to examine the health effects of net internal migration between relatively deprived and affluent areas
between 1971, 1981 and 1991 [4]. To control for initial poor-health selection, those who reported
being sick or disabled in 1971 were excluded from the analysis. In general they found that those who
were downwardly mobile had poorer health than their origin group, but better health than their
destination group. Conversely, those who went up the social ladder had health intermediate between
their group of origin and the more advantaged group they joined. Those who remained in the top
quintile across all censuses had the best health outcomes; and those who stayed in the bottom quintile
had the worst health.
These findings would suggest that selective migration would tend to reduce the (cross-sectional)
health gap between rich and poor areas. However the researchers found the opposite: selective
mobility increased the health gap [4-6].
The authors explored this apparent paradox further and showed that this was because of the
distribution in the relative numbers of those who moved into an area, those who moved out and those
who stayed put in the same quintile group. For example, in the most deprived quintile, mortality rates
increase because those moving out of the quintile (largest group) had better health than those who
moved into it (next biggest group); who in turn had better health than the 'stayers' (the smallest
group). In contrast, in the least deprived quintile, the out-migrants were the largest group and had
poorer health than both the new in-migrants and the stayers. The net effect of these moves therefore
resulted in the widening of the mortality inequalities between the most and least deprived quintiles.
Is the impact of selective migration material to cross-sectional analysis of trends in inequalities in
mortality? Norman and colleagues concluded that unlike health status measures such as limiting
long-standing illness, the impact of mobility on mortality was not significant and that the estimated
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flow is by relatively healthy people aged 20-59 moving from more to less deprived areas, rather than
the older age groups who are the focus of our research.
Over time, geographical patterns of inequality are maintained and often exaggerated. Where areas
change their level of deprivation or where people’s deprivation circumstances change, then there are
likely to be concomitant changes in health. The majority of change (as with social mobility) is in the ‘middle ground’ rather than in the extremes. Wholesale change is rare.
In summary: the majority of small areas in England have remained in their quintile group over
the 25-year period of our analysis. Furthermore, although selective migration of healthy people
to better-off areas is a factor, net migration would have some, but not a significant, impact on
the analysis of trends in inequalities we report.
References:
1. Norman P, Rees P & Boyle P (2003) Achieving data compatibility over space and time: creating consistent geographical zones. International Journal of Population Geography. 9(5): 365-386.
2. Gregory I (2009) Comparisons between geographies of mortality and deprivation from the 1900s and 2001: spatial analysis of census and mortality statistics. BMJ 339:b3454.
3. Norman P (2010) Identifying change over time in small area socio-economic deprivation. Appl Spatial Analysis and Policy 3: 107-138.
4. Norman P, Boyle P, Rees P (2005) Selective migration, health and deprivation: a longitudinal analysis. Social Science and Medicine 60: 2755-71.
5. Boyle P, Norman P (2009) Chapter 19: ‘Migration and Health’. In T Brown, S McLafferty, C Moon eds. A companion to health and medical geography. Wiley-Blackwell, Oxford, UK.
